Here's What Went Wrong with Last Year's Flu Vaccine

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Americans got little benefit from last season's flu shot — the
vaccine was only about 19 percent effective, according to the
Centers for Disease Control and Prevention. That's largely
because one of the flu strains that was used to make the vaccine
did not match well with the actual flu strains that were
circulating.

Now, new research shows that a single mutation in that strain is
what caused
this poor match.

In the study, researchers used blood from ferrets and sheep that
had been infected with a live version of the
H3N2 virus. (A killed version of this virus was used in the
flu vaccine.) They tested the antibodies the animals produced
against an array of flu viruses that contained different
mutations.

The researchers identified one mutation that when present, made
the animals' antibodies respond much more weakly to the
viruses.There was as much as a fourfold decrease in how effective
the antibodies were in fighting the virus that had this mutation,
the researchers said.

Subsequent experiments using blood from people who had received
the seasonal
flu vaccine produced similar results, according to the study
published today (June 25) in the journal Cell Reports.

"There was only one mutation that really had a profound effect,"
said Scott Hensley, the lead researcher on the study and an
assistant professor at the Wistar Institute, a medical research
center in Philadelphia. The mutation was found in the gene for
the virus's hemagglutinin molecule, the "H" part of the H3N2
strain.

Since the researchers already knew the vaccine was a bad match
this year, looking back and identifying the problem was the easy
part, Hensley said. "What we'd like to do is be able to look
ahead and try to predict how the virus might mutate in the
future,and be able to make predictions if next year's flu vaccine
will be effective," he told Live Science. But that is much harder
to do. [ The
9 Deadliest Viruses on Earth ]

Unlike some viruses, influenza's genetic code regularly mutates
as it replicates, so the virus is continuously changing; flu
experts refer to this process as antigenic drift. For this
reason, the flu vaccine needs to be created anew each year. In
February, the World Health Organization
recommended that the flu vaccine for the 2015-2016 flu season
include an updated strain of H3N2.

The new findings are important because they show researchers
where the "hot spots" in the virus are, which point to how the
virus mutates to get around the immune system, said Andrew
Pekosz, an influenza expert at the Johns Hopkins Bloomberg School
of Public Health who was not involved in the study.

Pekosz said that he's optimistic that next year's flu vaccine
will offer more protection against the H3N2 strain. But he's
concerned about another
flu strain, called H1N1, for next year's flu season.
Protection against H1N1 is also included in the seasonal flu
vaccine.

"The worry right now is the H1N1 component," he adds. "That virus
hasn't really changed in a very long period of time. Much longer
than we'd expect." That means, he says, that a change might be in
store for it.